clk: qcom: mdss: update PLL programming for DisplayPort on SDM855

Update the PHY bringup sequence for DisplayPort on SDM855 in
accordance with the latest documentation. Furthermore, update the
logic for the recalculation of the clock rate based on the
updated divider values for SDM855.

Change-Id: Ia1717b5852a710e2a8ee3a824b6d279fa0be3112
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>